1. Technical Field
This invention relates generally to amplifiers and, more particularly, to a Wheatstone bridge transducer amplifier circuit having integrated diagnostic testing circuitry for testing the amplifier circuit.
2. Discussion
Transducers are widely used to generate an electrical signal in response to a physical stimulus. For example, transducers can be designed to sense an amount of force due to pressure or motion/acceleration. Piezoresistive transducers are conventionally configured with strain sensing circuit elements fabricated on a semiconductor chip. Generally speaking, the strain sensing circuit is commonly configured as a Wheatstone bridge which is generally made up of four balanced resistors. Under balanced conditions, the Wheatstone bridge strain sensing circuit preferably exhibits equal value resistance with the resistor configuration and produces a zero differential voltage output. However, when subjected to an amount of strain caused by a physical stimulus, the resistive characteristics of the Wheatstone bridge change, thereby resulting in an unbalanced condition. At the same time, a measurable differential output voltage is produced across the differential outputs. The measured differential output is generally proportional to the physical stimulus being measured.
Since Wheatstone bridge transducers generally produce very small differential output signals, the differential output signals usually require amplification and signal compensation in order to provide output signals at useable amplitudes. The necessary amplification is usually accomplished with a specialized transducer amplifier. Additionally, it is often desirable to include the amplification circuitry in the same assembled mechanical package of the Wheatstone bridge strain sensing circuit so as to realize improved performance and lower cost. Furthermore, it is often necessary to test the characteristics of a transducer and amplifier circuit over time and use of the transducer circuit. This is because characteristics of the amplifier such as gain and offset are susceptible to change over time and use as should be evident to one skilled in the art. For example, a piezoresistive accelerometer transducer is preferably periodically tested when used in connection with automotive airbag applications. This is to ensure that the transducer is currently operating properly and that the amplifier is realizing the proper gain.
Once the transducer and amplifier are fully assembled in the mechanical package, diagnostic testing of the transducer and amplifier generally becomes much more complicated and very difficult to properly perform. This is because it is usually difficult or impracticable to adequately separate the Wheatstone bridge sensing circuit from the amplification circuitry to perform the necessary diagnostic testing. Also, with the assembled package it may not be possible to provide adequate physical stimulation to the Wheatstone bridge transducer as is often required to verify proper amplifier performance.
It is therefore desirable to provide a transducer amplifier circuit which may easily undergo diagnostic testing to ensure proper operation of the transducer and amplification circuitry.
More particularly, it is desirable to provide a balanced transducer amplifier circuit with an integrated diagnostic testing circuit which may receive a diagnostic test input signal and test the amplifier circuit in a diagnostic mode without affecting the normal operation of the amplifier circuit.
It is further desirable to provide a diagnostic test circuit which adds minimal circuitry for use with a Wheatstone bridge transducer coupled to a differential amplifier.
Yet, it is also desirable to provide for a method of applying a diagnostic test input signal to a differential input Wheatstone bridge amplifier to perform diagnostic testing of the amplifier circuit.